1. Field of the Invention
The present invention relates to a mechanical force sensor for detecting a mechanical force such as an acceleration, an angular acceleration, an angular velocity, or a load.
2. Description of the Related Art
An acceleration sensor including piezoelectric vibrators is disclosed in Japanese Unexamined Patent Application Publication No. 2002-243757 by the assignee of the present application. Japanese Unexamined Patent Application Publication No. 2002-243757 corresponds to U.S. patent application Ser. No. 10/054,858, filed Jan. 25, 2002, now pending.
In the acceleration sensor, a bridge circuit is defined by two piezoelectric vibrators to which stresses in mutually opposite directions are applied by an acceleration, and by load impedances including two capacitors. Furthermore, a voltage-dividing impedance circuit is provided between average outputs thereof, and a signal at the voltage-dividing point of the voltage-dividing impedance circuit is fed back to a node between the two piezoelectric vibrators by a feedback signal processing circuit, whereby an oscillation circuit is provided. A phase difference between oscillation outputs of the average outputs of the bridge circuit is detected and output as an acceleration detection signal.
(1) Factors of Variation in the Circuit Portion
The features of the acceleration sensor include the ability to detect an acceleration associated with a DC component such as gravitational acceleration, not being susceptible to the effect of electrostatic capacitance of the piezoelectric vibrators even when the resonant frequency is high, high sensitivity of detection, and no need to amplify a high-frequency voltage signal by a precise gain.
In the acceleration sensor, however, since a bridge circuit is defined by the two piezoelectric vibrators and the load impedances including two capacitors, the phase difference between oscillation outputs does not become zero unless the bridge becomes balanced. That is, even if stresses applied to the piezoelectric vibrators are both zero, the output of the acceleration sensor does not become zero. Furthermore, although it is an advantage that the load impedances can be adjusted to cancel out the variation between the piezoelectric vibrators when variation between the piezoelectric vibrators is large, if variation between the piezoelectric vibrators is so small from the start that adjustment is not needed, extra processing for equalizing the characteristics of the load impedances is required.
(2) Layout of the Piezoelectric Vibrators and the Circuit
The acceleration sensor, in which electrostatic capacitance is large relative to the load impedances, is not susceptible to the effect of stray capacitance of a circuit board. Thus, it does not require coating or hermetically sealed packaging.
In the acceleration sensor, however, the distance between the piezoelectric vibrators and the circuit must be kept within several cm. This is a constraint attributable to the use of the principle of Colpitts oscillator. That is, the constraint is attributable to the susceptibility to wiring resistance and inductance components due to high input impedance of the circuit and high input voltage to the circuit. Since it is assumed that the distance between the piezoelectric vibrators and the circuit will be 10 cm or longer in some applications, a need exists for a detection method that will work in principle even if the distance between the piezoelectric vibrators and the circuit is large as described above.
(3) Adjustment of Point of Maximal Sensitivity to Acceleration
The acceleration sensor allows sensitivity to acceleration to be adjusted to a maximal point by a phase-shift circuit. However, it is difficult to control the amount of phase shift by the phase-shift circuit.
The problems described above are not specific to acceleration sensors, but are common to cases where currents that flow through piezoelectric vibrators change in accordance with the amount of a mechanical force such as an angular acceleration, an angular velocity, or a load.
In order to solve the problems described above, preferred embodiments of the present invention provide a mechanical force sensor that eliminates the need for adjustment and reduces variation factors of a circuit portion by reducing variation in characteristics of two piezoelectric vibrators, and that allows the piezoelectric vibrators and the circuit portion to be disposed at a distance from each other, and that eliminates the need of controlling the amount of a phase shift by a phase-shift circuit.
According to a preferred embodiment of the present invention, a mechanical force sensor includes two piezoelectric vibrators to which stresses in mutually opposite directions are applied by a mechanical force, a circuit for applying a voltage signal commonly to the two piezoelectric vibrators, a current-voltage converter circuit for converting current signals that flow through the two piezoelectric vibrators into voltage signals, and a phase-difference signal processing circuit for detecting a phase difference between the output voltage signals of the current-voltage converter circuit and outputting a mechanical force detection signal. Accordingly, the effect on a measurement operation is minimized and stable measurement of a mechanical force is facilitated.
The mechanical force sensor may be such that the circuit for applying a voltage signal commonly to the two piezoelectric vibrators is a voltage amplifier circuit that provides the two piezoelectric vibrators with a positive feedback of a voltage signal from and in phase with a voltage signal obtained by summing the current signals that flow through the two piezoelectric vibrators, output from the current-voltage converter circuit, whereby the voltage amplifier circuit, the piezoelectric vibrators, and the current-voltage converter circuit causes an oscillating operation.
Accordingly, highly sensitive detection of a mechanical force is facilitated based on sharp responses of resonant frequencies to stresses applied to the two piezoelectric vibrators. Furthermore, characteristics of the two piezoelectric vibrators affect operating points of oscillating operations, thereby stabilizing change in oscillating frequency in relation to stresses applied to the two piezoelectric vibrators.
The mechanical force sensor may be such that the voltage amplifier circuit includes a voltage amplitude limiting circuit including a constant current circuit and a current switching circuit, and the voltage amplitude limiting circuit limits a voltage amplitude of the voltage signal commonly applied to the two piezoelectric vibrators. Accordingly, the driving voltage of the piezoelectric vibrators is maintained constant, stabilizing circuit operation and preventing generation of heat by the piezoelectric vibrators.
The mechanical force sensor may be such that a frequency of the oscillating operation is a frequency in a resonant frequency range of the piezoelectric vibrators Since the impedances of the piezoelectric vibrators are low in resonant frequency ranges thereof, large currents that flow into the current-voltage converter circuit to increase the gain, causing a stable oscillating operation This improves sensitivity to change in a signal to be detected in accordance with a mechanical force applied to the sensor.
The mechanical force sensor may be such that the resonant frequency range is a range in which admittance phases of the piezoelectric vibrators are within approximately 0xc2x145 degrees. Accordingly, change in the phase difference between the output voltage signals of the current-voltage converter circuit in accordance with a mechanical force applied becomes more linear, allowing detection of the mechanical force in a larger dynamic range.
The mechanical force sensor may be such that the current-voltage converter circuit includes two differential amplifier circuits for respectively generating current signals in opposite phases with input current signals to cancel out the input current signals and for respectively distributing the current signals in the opposite phases into two, and of the two distributed signals associated with each of the two differential amplifier circuits, first current signals flow through a common impedance element and second current signals flow respectively through different impedance elements, thereby generating voltage signals to be output.
By converting changes in the phase of currents that flow through the two piezoelectric vibrators into changes in voltages by two differential amplifier circuits as such, noise components in phase with each other (drift) are removed.
The mechanical force sensor may be such that at least one of the impedance elements is a resistor. Accordingly, cost is reduced, and voltage signals that are in phase with the currents that flow through the two piezoelectric vibrators are obtained, readily allowing a positive feedback to the piezoelectric vibrators by the voltage amplifier circuit.
In the mechanical force sensor, the differential amplifier circuits may be arranged such that emitters or sources of first and second transistors are connected to each other, a first resistor is connected between a node therebetween and an analog ground, emitters or sources of third and fourth transistors are connected to each other, a second resistor is connected between a node therebetween and the analog ground, bases or gates of the first to fourth transistors are connected to a constant voltage source, collectors or drains of the second and third transistors are connected to each other, a fifth resistor is connected between the collectors or drains and a power supply line, and third and fourth resistors are connected between the collectors or drains of the first and fourth transistors and the power supply line, respectively.
By forming common-base amplifier circuits or common-gate amplifier circuits as described above, input impedances of the amplifier circuits are decreased and input capacitances are reduced, so as to provide voltage amplifier circuits and oscillation circuits having favorable frequency characteristics.
The mechanical force sensor may be such that the phase-difference signal processing circuit is a differential phase difference-voltage converter circuit that receives a differential input of the output voltage signals of the current-voltage converter circuit and that outputs a voltage signal representing a phase difference. Accordingly, noise components in phase with each other are removed, so that a voltage signal in accordance with a mechanical force, having small noise components on the whole, is obtained.
The mechanical force sensor may be such that resistors are connected respectively in series with the two piezoelectric vibrators. Accordingly, sensitivity of detection in response to stresses applied to the piezoelectric vibrators is stabilized, and temperature characteristics are compensated.
The mechanical force may be, for example, an acceleration, an angular acceleration, an angular velocity, or a load.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings